1. Field of the Invention
This invention relates to the reclamation of molybdenum from spent catalyst materials, and more specifically, preferably relates to reclamation of molybdenum as molybdenum trioxide.
2. Description of the Prior Art
Spent catalyst materials typically consist of the original catalyst materials (commonly including compounds of molybdenum, tungsten, aluminum, cobalt, nickel, and sulfur) that have impaired catalytic activities due to contamination with materials, such as carbon, iron, vanadium, arsenic, silica, etc.
The value of the molybdenum contained within spent catalyst materials has made reclamation of molybdenum from these materials an important source of molybdenum, resulting in the development of a number of processes to recover this valuable commodity.
Many of these recovery processes, such as those described in U.S. Pat. Nos. 2,367,506; 3,773,890; 3,957,946; 4,046,852; 4,495,157; 5,702,500; 6,149,883; and 7,169,371; are based on the hydrometallurgical extraction of molybdenum, and are typically composed of a three-step process where:
The spent catalyst is roasted or partially gasified (to remove coke and hydrocarbons; typically under a wet atmosphere between 850° F. to 1650° F.) typically with an alkali salt to yield a soluble molybdenum compound. (This step is generally at atmospheric pressure, but can be performed at elevated pressures.)
The residue is extracted by liquid-solid leaching or digestion to dissolve molybdenum from the residue to form molybdenum-containing liquor.
The molybdenum is either precipitated or extracted from the molybdenum-containing liquor.
Subsequent steps in these processes are specific to the process and are largely dictated by the desired form of the final molybdenum product.
Other recovery processes utilize the carbothermal reduction of spent catalyst materials at elevated temperatures under a reducing atmosphere, such as described by U.S. Pat. No. 4,384,885, as well as smelting in an electric arc furnace, such as described by U.S. Pat. No. 4,349,381.
While some of these existing recovery processes have proven to be successful in the recovery of molybdenum, they may not be entirely satisfactory due to the expensive and/or large quantities of chemical reagents required to realize the recovery of the molybdenum. Likewise, these processes may also be hampered by the elaborate and costly processing equipment required to process large quantities of spent catalyst materials.
With the exception of the carbothermal reduction process, all of the previously mentioned processes are hampered by presence of carbon in the spent catalyst, which is present in significant quantities in most molybdenum-containing spent catalyst. The processes that are hampered by the presence of carbon all utilize techniques that remove the carbon that interferes with the winning of the molybdenum from the spent catalyst materials.
Another similar characteristic of the previously mentioned processes (especially the hydrometallurgical processes that focus on complete separation of molybdenum from other metal species) is how the presence of tungsten in spent catalyst materials can interfere with the recovery of molybdenum without tungsten contamination. This is due to molybdenum and tungsten, both being periodic Group 6 elements, having similar chemical potential, and the tendency to form heteropolymeric species within aqueous solutions. While processes and techniques that can affect the separation of tungsten or molybdenum from pregnant liquors exist (e.g., U.S. Pat. No. 3,969,478), the relatively low value of the tungsten can make this endeavor uneconomical.
The prior art of molybdenum reclamation also includes the reclamation of molybdenum from spent catalysts used to catalyze epoxidation reactions. These types of molybdenum containing catalysts are essentially liquid, organic residues composed mainly of alkylene glycols distilled from unreacted olefinic and organic hydroperoxide compounds. Reclamation of the molybdenum from these types of spent catalysts is detailed in U.S. Pat. Nos. 4,455,283 and 5,503,813. Since these spent catalyst materials typically contain significant amounts of alkalis, reclamation of molybdenum by the techniques described in this invention would be impractical (due to the alkali-induced slagging) unless the alkali materials had been removed from the spent epoxidation catalyst materials.
The prior art of molybdenum reclamation is often considered to include a variety of roasting processes where molybdenum-containing materials (usually molybdenum ores, such as molybdenite or other molybdenum disulfide-rich materials) are heated in a controlled, oxidizing atmosphere in an apparatus, such as a Herreshoff roasting furnace as described in U.S. Pat. No. 1,085,419, to affect the oxidation of the molybdenum sulfide, yielding molybdenum oxides (commonly the molybdenum dioxide, molybdenum trioxide, or some other suboxide with an average oxygen content that is between these oxide species). The sulfur is converted to sulfur dioxide vapor that exits the roaster while the resulting oxide residue is kept at temperatures below 1300° F. to prevent the fusion, melting, or partial sublimation of this material in the roaster. This type of molybdenum ore roasting process is described in U.S. Pat. Nos. 4,758,406 and 3,833,352 (with an interesting variation described in U.S. Pat. No. 4,552,749 that utilizes the molybdenum trioxide vapor as the oxidant for the molybdenum disulfide) and should not be considered as molybdenum reclamation processes, in a true sense, because these processes do not separate the molybdenum containing species from the other contaminants or gangue that may be coincident with the feedstock to these processes. For example, compounds of copper, tungsten, nickel, and silicon are common impurities in molybdenum ore roasting feedstocks, but these compounds are not removed from the molybdenum oxide product as a result of the roasting process.
The prior art of molybdenum reclamation may also be extended to consider processes where a feedstock that already contains molybdenum trioxide is heated in an atmosphere to affect the separation of the contained molybdenum trioxide as a vapor from the other constituents contained in the feedstock that remain in a condensed (i.e., non-vapor) phase. An example of such separation is demonstrated in U.S. Pat. No. 4,551,313, where an entrained flow of granular molybdic oxide-containing feedstock is mixed with a fuel gas and an oxidizing gas, with the entire mixture introduced into a furnace chamber. This gas/solid mixture is heated to a temperature between 2900° F. and 3200° F. in order to sublime the molybdenum trioxide and slag (i.e., melt) the remaining, impure fraction of the feedstock. The impurities from the feedstock are collected in a slag pool or flow into a slag pot, with the whole invention relying, in part, on the melting of slag-forming constituents in the feedstock to aid in the capture of non-volatile material that is suspended in the vapor stream.
Another prior art relating to the reclamation of molybdenum incorporates the oxidation of molybdic sulfide materials (especially ores, such as molybdenite) along with subsequent sublimation of the produced molybdenum trioxide at elevated temperatures. Examples of processes that employ of this type of reclamation are demonstrated by several United States patents, i.e., U.S. Pat. Nos. 1,426,602; 3,139,326; and 4,555,387.
In U.S. Pat. No. 1,426,602, Robertson describes a process and apparatus for the oxidation of molybdenite, followed by the sublimation of molybdenum oxide in a circulating gas circuit which cyclically heats the molybdenum oxide and subsequently cools the gas for condensation in various points along the circuit. No operating temperatures are cited in the patent, and the practicality of the invention is questionable, since the deposition of solid molybdenum oxides along various points in the circuit will likely result in the fouling and blockage of the circuit.
U.S. Pat. No. 3,139,326 discloses finely divided molybdenite is oxidized and sublimed in a conventional box furnace operating at temperatures of at least 1800° F. and then quenched to 2200° F. as it exits the furnace. Separation of the residues from the molybdenum oxide vapor is accomplished with a series of ceramic baffles within the furnace and by subsequent filtration of the molybdenum trioxide vapor by a ceramic-fiber filter assembly. As a consequence of choosing to filter the molybdenum trioxide in its vapor state, both a maximum and minimum operating temperature constraints are imposed on the filtering of this vapor stream. First, there is the maximum operating temperature limitation that is imparted by the ceramic filter media since even the most refractory of filter media do not commonly operate at temperatures in excess of 2200° F. Second, a minimum operating temperature is imparted by the thermophysical properties of molybdenum trioxide, requiring that temperatures of the vapor stream be sufficiently high to prevent the deposition of the molybdenum trioxide on the filter media. Since this particular furnace design operates at or near atmospheric pressure, a temperature of at least 1800° F. must be maintained to prevent all but the most dilute vapor streams of molybdenum trioxide from condensing. Thus, the operating window of this filter assembly is restricted to 1800° F. to 2200° F. In other embodiments of this invention, the molybdenum trioxide vapor stream is to be filtered at 1350° F. to affect the separation from the nonvolatile compounds. While this is possible, this low temperature imparts significant limitations on the maximum volume fraction of the molybdenum trioxide in the vapor phase since the vapor pressure of molybdenum trioxide at 1350° F. is only on the order of 0.001 atmospheres. Maintaining such low concentrations of molybdenum trioxide in the vapor stream seriously impacts the operating efficiencies and intensities of this process.
In U.S. Pat. No. 4,555,387, the process is essentially the same as that described in an earlier patent by the same inventors (U.S. Pat. No. 4,551,313), referenced above, as a process for the simple purification of molybdenum trioxide. In these more recent embodiments of their invention the only difference is that the molybdenum trioxide feedstock is replaced with a feedstock composed of a molybdenum sulfide concentrate. In this invention, as before, an entrained flow of granular material (now a molybdenum sulfide-containing feedstock) is mixed with a fuel gas and an oxidizing gas, with the entire mixture introduced into a furnace chamber. This gas/solid mixture is heated to a temperature somewhere between 2900° F. and 3200° F. in order to sublime the molybdenum trioxide and slag (i.e., melt) the remaining, impure fraction of the feedstock. The impurities from the feedstock are collected in a slag pool or flow into a slag pot, with the whole invention relying, in part, on the melting of slag-forming constituents in the feedstock to aid in the capture of non-volatile material that is suspended in the vapor stream.
The present invention overcomes many of the complications and disadvantages of the prior art in molybdenum recovery processes by providing a relatively simple, efficient, and intensive (i.e., high throughput to reactor area ratio) process that relies on the thermophysical properties of molybdenum trioxide, and is unaffected by the interference of carbon and tungsten that may be present in spent catalyst materials.